C++ 20 协程 之 Hello World（一）
C++ 20 协程 之 Hello World
C++ 20 协程
C++协程可以提高资源利用率，利用状态机解决回调地狱的问题。与Kotlin协程具有相似之处，但是使用难度较为繁琐。
Hello World 非挂起
C++协程最为简单的Hello World案例如下:
#include <coroutine>
#include <iostream>
struct HelloCoroutine{
HelloCoroutine() {
std::cout << "HelloCoroutine constructed\n";
}
~HelloCoroutine() {
std::cout << "HelloCoroutine destructed\n";
}
struct promise_type{
promise_type(){
std::cout << "promise_type constructed\n";
}
~promise_type(){
std::cout << "promise_type destructed\n";
}
HelloCoroutine get_return_object(){
std::cout << "get_return_object\n";
return {};
}
std::suspend_never initial_suspend(){
std::cout << "initial_suspend\n";
return {};
}
std::suspend_never final_suspend() noexcept {
std::cout << "final_suspend\n";
return {};
}
void return_void(){
std::cout << "return_void\n";
}
void unhandled_exception(){
std::cerr << "Coroutine encountered an exception.\n";
}
};
};
HelloCoroutine helloCoroutine(){
std::cout << "Hello, Coroutine!" << std::endl;
// throw std::runtime_error("Coroutine cannot be suspended initially."); // 1
co_return;
}
void learn01(){
helloCoroutine();
std::cout << "This is learn01 function." << std::endl;
}
/*
输出如下：
promise_type constructed
get_return_object
HelloCoroutine constructed
initial_suspend
Hello, Coroutine!
return_void
final_suspend
promise_type destructed
HelloCoroutine destructed
This is learn01 function.
============
打开1处的注释的执行结果如下:
promise_type constructed
get_return_object
HelloCoroutine constructed
initial_suspend
Hello, Coroutine!
Coroutine encountered an exception.
final_suspend
promise_type destructed
HelloCoroutine destructed
This is learn01 function.
*/
分析一下，根据返回结果可以推测出helloCoroutine被转换为以下函数：
HelloCoroutine helloCoroutine1(){
coroutine_frame *frame = new coroutine_frame();
HelloCoroutine::promise_type &promise = frame->promise;
HelloCoroutine return_object = promise.get_return_object(); // init HelloCoroutine
co_await promise.initial_suspend();
try{
std::cout << "Hello, Coroutine!" << std::endl;
promise.return_void(); // co_return;
}catch(...){
promise.unhandled_exception();
}
co_await promise.final_suspend();
}
Hello World 挂起
struct HelloCoroutine{
public:
struct promise_type{
promise_type(){
std::cout << "promise_type constructed\n";
}
~promise_type(){
std::cout << "promise_type destructed\n";
}
HelloCoroutine get_return_object(){
std::cout << "get_return_object\n";
return HelloCoroutine(std::coroutine_handle<promise_type>::from_promise(*this));
}
std::suspend_always initial_suspend(){
std::cout << "initial_suspend\n";
return {};
}
std::suspend_never final_suspend() noexcept { // 1
std::cout << "final_suspend\n";
return {};
}
void return_void(){
std::cout << "return_void\n";
}
void unhandled_exception(){
std::cerr << "Coroutine encountered an exception.\n";
}
};
void resume(){
if(!handle.done()){
handle.resume(); // 2
}
}
HelloCoroutine(std::coroutine_handle<promise_type> coroutineHandle) : handle(coroutineHandle) {
std::cout << "HelloCoroutine constructed\n";
}
~HelloCoroutine() {
std::cout << "HelloCoroutine destructed\n";
// handle.destroy(); 3
}
private:
std::coroutine_handle<promise_type> handle; // 指向协程帧
};
HelloCoroutine helloCoroutine(){
std::cout << "Hello, Coroutine!" << std::endl;
co_return;
}
void learn01(){
HelloCoroutine hello = helloCoroutine();
hello.resume();
hello.resume();
std::cout << "This is learn01 function." << std::endl;
}
/*
执行结果
promise_type constructed
get_return_object
HelloCoroutine constructed
initial_suspend
Hello, Coroutine!
return_void
final_suspend
promise_type destructed
This is learn01 function.
HelloCoroutine destructed
*/
代码分析
使用co_return的函数在调用时会在堆上创建coroutine_frame对象。该对象会保持以下数据
- 局部变量
promise_type- 状态机信息
源码查看：
// 17.12.5 Trivial awaitables
/// [coroutine.trivial.awaitables]
struct suspend_always
{
constexpr bool await_ready() const noexcept { return false; }
constexpr void await_suspend(coroutine_handle<>) const noexcept {}
constexpr void await_resume() const noexcept {}
};
struct suspend_never
{
constexpr bool await_ready() const noexcept { return true; }
constexpr void await_suspend(coroutine_handle<>) const noexcept {}
constexpr void await_resume() const noexcept {}
};
所以首先执行promise_type的构造函数。之后调用get_return_object创建HelloCoroutine对象。之后调用initial_suspend返回std::suspend_always代表永久阻塞，当耗时任务执行完毕，必须调用handle的resume方法恢复协程，在复杂应用中，为防止被多次resume程序奔溃，使用handle的done方法判断是否已经resume过,没有被resume过，则resume。紧接着调用
helloCoroutine的方法体，有异常的话执行unhandled_exception处理异常，没有异常正常执行，之后走到co_return;语句，则执行return_void方法处理返回逻辑。接着执行final_suspend方法结束协程，当返回std::suspend_never代表不挂起协程，直接结束协程，则立马销毁协程帧、HelloCoroutine对象。当返回std::suspend_always代表挂起协程，必须在HelloCoroutine对象的析构函数中调用handle的destroy方法销毁协程帧率对象，否则内存泄漏。
Hello World 返回内容
struct HelloCoroutine{
public:
struct promise_type{
private:
int value; // 用于存储协程的返回值, 默认0
public:
promise_type(){
std::cout << "promise_type constructed\n";
}
~promise_type(){
std::cout << "promise_type destructed\n";
}
HelloCoroutine get_return_object(){
std::cout << "get_return_object\n";
return HelloCoroutine(std::coroutine_handle<promise_type>::from_promise(*this)); // 解引用
}
std::suspend_always initial_suspend(){
std::cout << "initial_suspend\n";
return {};
}
std::suspend_always final_suspend() noexcept {
std::cout << "final_suspend\n";
return {};
}
void return_value(const int value){
this->value = value;
std::cout << "return_void with value: " << value << "\n";
}
void unhandled_exception(){
std::cerr << "Coroutine encountered an exception.\n";
}
int getValue(){
return value;
}
};
int getValue() const { return handle.promise().getValue(); }
void resume(){
if(!handle.done()){
handle.resume();
}
}
HelloCoroutine(std::coroutine_handle<promise_type> coroutineHandle) : handle(coroutineHandle) {
std::cout << "HelloCoroutine constructed\n";
}
~HelloCoroutine() {
std::cout << "HelloCoroutine destructed\n";
handle.destroy();
}
private:
std::coroutine_handle<promise_type> handle; // 指向协程帧
};
HelloCoroutine helloCoroutine(){
std::cout << "Hello, Coroutine!" << std::endl;
co_return 520;
}
void learn01(){
HelloCoroutine hello = helloCoroutine();
hello.resume();
std::cout << "Coroutine returned value: " << hello.getValue() << "\n";
}
/*
执行结果如下：
promise_type constructed
get_return_object
HelloCoroutine constructed
initial_suspend
Hello, Coroutine!
return_void with value: 520
final_suspend
Coroutine returned value: 520
HelloCoroutine destructed
promise_type destructed
*/
Hello World 生成器
struct HelloCoroutine {
public:
struct promise_type {
private:
int value; // 用于存储协程的返回值, 默认0
public:
promise_type(){
std::cout << "promise_type constructed\n";
}
~promise_type(){
std::cout << "promise_type destructed\n";
}
HelloCoroutine get_return_object(){
std::cout << "get_return_object\n";
return HelloCoroutine(std::coroutine_handle<promise_type>::from_promise(*this)); // 解引用
}
std::suspend_always initial_suspend(){
std::cout << "initial_suspend\n";
return {};
}
std::suspend_always final_suspend() noexcept {
std::cout << "final_suspend\n";
return {};
}
std::suspend_always yield_value(int value){
this->value = value;
std::cout << "yield_value with value: " << value << "\n";
return {};
}
void unhandled_exception(){
std::cerr << "Coroutine encountered an exception.\n";
}
int getValue(){
return value;
}
};
int getValue() const { return handle.promise().getValue(); }
HelloCoroutine(std::coroutine_handle<promise_type> coroutineHandle) : handle(coroutineHandle) {
std::cout << "HelloCoroutine constructed\n";
}
~HelloCoroutine() {
std::cout << "HelloCoroutine destructed\n";
handle.destroy();
}
bool next() const {
if(!handle.done()){
handle.resume();
}
return !handle.done();
}
private:
std::coroutine_handle<promise_type> handle; // 指向协程帧
};
HelloCoroutine helloCoroutine(){
std::cout << "Hello, Coroutine!" << std::endl;
for(int i = 0; i < 5; ++i){
co_yield i; // yield_value
}
}
void learn01(){
HelloCoroutine hello = helloCoroutine();
while(hello.next()){
std::cout << "Coroutine yielded value: " << hello.getValue() << "\n";
}
}
TODO co_await, thread_pool, thread switch
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